Stable lead alkyl compositions and a method for preparing the same



United States Patent 2,992,253 STABLE LEAD ALKYL COMPOSITIONS AND A METHOD FOR PREPARING THE SAME Hymin Shapiro, Baton Rouge, La., and Herbert R. Neal, Farmington, Mich., assignors to Ethyl Corporation, New York, N.Y., a corporation of Delaware No Drawing. Filed Dec. 8,1959, Ser. No. 858,017 4 Claims. (Cl. 260437) This invention relates to alkyllead compositions which are stable at temperatures above 100 C. It also relates to methods for inhibiting the thermal decomposition of alkyllead compounds when subjected to temperatures above 100 C., at which temperature thermal decomposition becomes appreciable.

Generally this invention contemplates inhibiting the thermal decomposition of alkyllead compounds in which at least one valence of the lead is satisfied by an alkyl radical.

More specifically, this invention is concerned with an improved process for separating alkyllead compounds from the reaction products accompanying their synthesis. It is also applicable to a method for inhibiting thermal decomposition of an alkyllead product during its purification and blending with other products in making commercial antiknock fluids. It is applicable to minimizing the possibility of thermal decomposition during storage or transportato-n of an alkyllead product. It is especially applicable to preventing thermal decomposition of undiluted alkyllead compounds where the likelihood of thermal decomposition is more of a problem.

As is well known, tetraalkyllead antiknook compounds generally are produced by reacting a sodium-lead alloy with an alkyl halide. Due to recent marked improvements in the technology of alkyllead manufacture, thermal instability of alkyllead compounds during synthesis is no longer a problem. However, the tetraalkyllead compound so produced is in admixture with various reaction by-products from which it must be separated. Separation is etlected by steam or vacuum distillation with subsequent purification of the tetraalkyllead distillate. Due to the toxic and unstable nature of tetraalkyllead antiknock compounds, these distillation and purification operations are subject to many difliculties.

In these distillation and purification operations meticulous temperature control and exact safety measures are of paramount importance. The rate of decomposition of the alkyllead compound increases rapidly with small rises in temperature above the temperature where thermal decomposition becomes appreciable. For example, decomposition of tetraethyllead occurs at the rate of approximately 2 percent per hour at a temperature of 100 C., which is the customary temperature used in separating tetraethyllead from the reaction products accompanying its synthesis. At temperatures above 100 C., the decomposition rate increases logarithmically so that a point is soon reached Where external heat is no longer required and decomposition becomes self-propagating.

Such likelihood of excessive decomposition is present also during blending, handling, storage, and transportation. Prior to diluting the alkyllead concentrate with scavengers, gasoline or other materials, the alkyllead compound remains a concentrate and the problem of excessive thermal decomposition exists, even though the temperature is maintained normally well below that of decomposition. For example, in the purification step wherein the tetraethyllead concentrate is washed and blown with air at atmospheric temperature to remove impurities, a sudden increase in temperature may occur due to the oxidation of triethylbismuth which is present as an impurity. Also pumps used in handling tetraethyl- Patented July 11, 1961 ICC lead occasionally freeze and the friction developed may cause a local overheating to a temperature above the temperature of decomposition of the tetraethyllead. Faulty wiring, leaks onto steam pipes, and other accidental causes also may produce local overheating with resulting dangerous decomposition.

Hence, in operations where alkylleads are in the undiluted or concentrated state-separation, PllllfiCElllOl'l, blending, transportation, and storage-the likelihood of thermal decomposition must be provided for and eifectively combatted.

Our object is to stabilize alkyllead compounds against thermal decomposition during these operations: separation, purification, blending, transportation, and storage.

This object is achieved by incorporating with alkyllead compounds a small quantity of a material capable of inhibiting alkyllead thermal decomposition. This object is also achieved by conducting each of the above operations in the presence of such a material. The materials which have been found to possess this property are referred to below as thermal stabilizers.

The thermal stabilizers of this invention are thiovanic acid (i.e. Z-mercaptoethanoic acid) and the esters thereof. While this invention will be discussed with particular reference to thiovanic acid itself, the esters thereof are equivalent in all respects thereto for the purposes of this invention. These thermal stabilizers when used in amounts varying from about 0.5 to about 10 percent by weight of the lead alkyl product are eiiective in substantially retarding or preventing thermal decomposition at temperatures above C. for extended periods of time.

The above thermal stabilizers are, in general, stable under the conditions to which alkyllead compounds may be subjected during their separation, purification, blending, transportation, and storage. For example, the above thermal stabilizers have no tendency to react to form gums or other obnoxious reaction products in the alkyllead composition.

The main thermal decomposition products of alkylleads are lead metal and hydrocarbon gas. Thus, a very good index of alkyllead thermal decomposition is gas liberation.

To illustrate the eifectiveness of this invention, direct comparisons were made between the decomposition characteristics of unstabilized and stabilized tetraethyllead. A thermostatically controlled hot oil bath was fitted with a stirrer, thermometer, and a holder for a small reaction tube. A 100 cc. gas buret beside the bath, and equipped with a water-containing levelling bottle, was connected by means. of rubber tubing with the reaction tube after the desired sample was introduced into this tube. After the bath was brought to a steady temperature of 160 C., the sample-containing tube was quickly immersed in the bath and clamped with the levelling bottle adjusted to hold the gas buret in place at a zero reading. Then measured was the time during which the sample was held at 160 C. without pronounced thermal decomposition and consequent gas evolution occurring. Thus, the longer the time, the more thermally stable was the alkyllead composition.

With pure tetraethyllead used in 1 ml. amounts, pronounced thermal deterioration occurred within 1 minute as evidenced by rapid gas evolution. In fact, the decomposition of unstabilized tetraethyllead will normally become uncontrollable if it is heated, whether rapidly or slowly, to even C., unless it is possible to very rapidly cool it down to about 100 C. or below.

However, when to the same amount of tetraethyllead there was previously added 2 percent by weight of thiovanic acid, no pronounced thermal deterioration occurred for over 50 minutes. Sustitution of various esters of thiovanic acid such as the methyl-, ethyl-, propyl-, isopropyl-,

clave. For example, one of the above thermal stabilizers may be added in appropriate quantity to the alkyllead reaction concentrate just before the separation step which is conducted at a temperature close to the temperature where hazardous run-away decomposition is particularly 7 prevalent. By adding one of the above thermal stabilizers to the reaction concentrate just prior todistillation, the danger arising from unexpected temperature increases is substantially eliminated.

Most preferably the above thermal stabilizers are employed to stabilize the alkyllead compound both in storage and in shipping and especially to stabilize any alkyllead concentrate, i.e., compositions containing at least 80 per cent by weight of alkyllead compound. If elevated temperature conditions are likely to be encountered, the addition of a small amount of thermal stabilizer to the alkyllead compound will economically and satisfactorily eliminate most of the hazard involved. While meticulous temperature control and exacting safety measures have been successful in reducing to a minimum the hazards of processing and handling of tetraethyllead, the use of this invention provides .a much greater factor of safety. Fur thermore, waste of the alkyllead product due to decomposition is considerably minimized through the use of this invention.

This invention is useful in stabilizing alkyllead compounds in which at least one valence of the lead is satisfied by an alkyl radical. For example tetraethyllead, tetrarnethyllead, tetrapropyllead, dimethyldiethyllead, triethylphenyllead, and t-riethyllead bromide can be successfully stagilized against thermal decomposition by incorporating therein a relatively small quantity of one of the thermal stabilizers of this invention.

We claim:

1. A method of inhibiting the decomposition of an alkyllead compound at temperatures from about. C. to about C. which comprises incorporating with said compound a small amount of thiovanic acid sufficient to inhibit such decomposition.

2. In the process of producing an alkyllead compound by reacting a sodium lead alloy with alkyl chloride and separating the thus produced lead alkyl compound from the reaction mass by steam distillation, the step which comprises conducting said steam distillation in the presence of thiovanic acid sufficient to inhibit thermal decomposition of the alkyllead compound.

3. An alkyllead compound containing, in amount sufficient to inhibit thermal deterioration thereof at temperatures from about 100 C. to about 160 C., thiovanic acid.

4. The composition of claim 3 wherein said 'alkyllead compound is tetraethyllead.

References Cited in the file of this patent UNITED STATES PATENTS 2,660,592. Calingaert Nov. 24, 1953 2,660,593 Calingaert Nov. 24, 1953 2,660,594 Calingaert Nov. 24, 1953 2,727,053 Krohn Dec. 13, 1955 

1. A METHOD OF INHIBITING THE DECOMPOSITION OF AN ALKYLLEAD COMPOUND AT TEMPERATURES FROM ABOUT 100*C. TO ABOUT 160*C. WHICH COMPRISES INCORPORATING WITH SAID COMPOUND A SMALL AMOUNT OF THIOVANIC ACID SUFFICIENT TO INHIBIT SUCH DECOMPOSITION. 